<!-- <!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        /* 层级选择器 */
        /* .box1 h1{color: red;}
        .box2 h1{font-size: 30px;} */
        /* 组合选择器 */
        /* .box1 h1,.box1 h2{color: red;} */
        /* 伪类选择器 */
        .box{
            width: 100px;
            height: 100px;
            border-color: rgb(110, 13, 255);
        }
        .box:hover{
            border-color: rgb(225, 20, 20);
        }
    </style>
</head>
<body>
<div class="box"></div>
</body>
</html> -->
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
<style>
    .box{width: 100px;
        height: 100px;
        background-color: red;
    }
    .box:hover{
        width: 100px;
        height: 100px;
        background-color: blue;
    }
</style>
</head>
<body>
<div class="box"></div>
</body>
</html>